The present invention relates in general to television receivers and more particularly to a novel circuit arrangement for recognizing a properly tuned video intermediate-frequency (IF) subcarrier signal.
So-called television signal identification or recognition systems find use in a variety of applications in television receiver tuning systems. In particular, signal seeking tuning systems and wide band automatic frequency control (AFC) systems frequently utilize such signal recognition systems. In signal seeking tuning systems, for example, the television tuner is typically caused to scan the RF television frequency spectrum until a received signal is recognized as comprising a valid television signal whereupon a control signal is developed inhibiting further scanning. An exemplary system of the foregoing type is disclosed in U.S. Pat. No. 3,737,565 to Ma et al. Wide band AFC systems, on the other hand, sometimes utilize signal recognition arrangements to facilitate intra-channel tuning in association with a conventional narrowband AFC circuit. U.S. Pat. No. 3,949,158 to Rzeszewski et al typifies a wide band AFC system of the latter type. In either case, the primary function of the signal recognition system is to interrogate a television channel, or part of a television channel, to identify the condition wherein a received television signal has been properly tuned by the receiver. Proper tuning in this sense, of course, contemplates the production of a video IF subcarrier signal at the video IF frequency of the receiver.
Conventionally, television signal recognition systems employ a tuned circuit, resonant at the video IF subcarrier frequency, for developing a recognition signal in response to resonance of the tuned circuit. In such systems, it is generally assumed that if a signal is detected by the resonant circuit, the detected signal is a video subcarrier and that therefore the receiver is receiving and is properly tuned to a broadcast television signal. However, under various circumstances, it is possible for the tuner to be mistuned or for the broadcast television signal to be offset from its assigned frequency so that, for example, the lower adjacent channel audio subcarrier beats with the tuner local oscillator to produce a signal having a frequency corresponding to the video IF frequency of the receiver. Under these conditions, the tuned circuit is caused to resonate in response to the adjacent channel audio subcarrier and to consequently develop a recognition signal even though the receiver is actually mistuned. To overcome this problem, prior art recognition systems generally employ additional circuitry to more reliably associate only a received picture carrier with the video IF frequency.
For instance, in U.S. Pat. No. 3,737,565 to Ma et al, a recognition system is disclosed employing tuned circuits resonant at the video and audio IF frequencies as well as circuitry for detecting the presence of horizontal synchronizing signals. Outputs from the tuned circuits and from the sync detector are coupled to a coincidence gate for producing a recognition signal. Systems utilizing circuits tuned to the video and audio IF subcarriers but detecting vertical sync in lieu of horizontal sync are disclosed in U.S. Pat. Nos. 3,949,158 and 4,041,535 both issued to Rzeszewski et al. A somewhat different approach is taught in U.S. Pat. No. 3,916,093 to Parker wherein signal recognition is effected in response to the comparative frequencies of the received video and audio IF signals. Finally, to provide a system independent of video signal strength, U.S. Pat. No. 3,825,838 to Mayle teaches the use of a tuned circuit resonant at the video IF frequency only during the concurrence of a horizontal synchronizing pulse and a horizontal flyback pulse.
It will be observed that most of the prior art systems discussed above rely on the utilization of means to detect the presence of a signal carrier at the IF audio frequency in addition to the detection of a carrier signal at the video IF frequency. As mentioned previously, this is done to more reliably develop a recognition signal only in response to a video subcarrier having a frequency corresponding to the video IF frequency. A problem encountered in several situations, however, is that the television signal supplied to the tuner simply contains no audio information thereby rendering such systems largely ineffective. For example, audio information is not infrequently deleted from television signals coupled to the receiver from master antenna television systems and video games. Moreover, for the situation where the tuner is mistuned so that the adjacent channel audio subcarrier is locked at the video IF frequency, third harmonics of the adjacent audio subcarrier may be produced and these harmonics will occur at a frequency normally expected to be occupied by the channel's associated audio carrier. As a result, the IF frequency spectrum is characterized by the adjacent channel audio subcarrier at the video IF frequency and its third harmonics at the audio IF frequency. Now, since subcarrier signals are present at both the video and audio IF frequencies, the prior art recognition system may be "fooled" into developing a recognition signal even though the channel is not properly tuned.
In addition, monitoring the additional parameter of horizontal or vertical sync does not entirely avoid the foregoing problems. More specifically, cross and intermodulation effects frequently cause the erroneously tuned adjacent channel audio carrier to exhibit sync signal components which are generally not discernable by prior art recognition systems from the sync signals present in the video signals. Consequently, a recognition signal may again be developed even though the receiver is not properly tuned to a television channel.